Studies on some strains of tobacco mosaic virus in orchids. I. Biological, chemical, and serological studies.

Seven isolates (01-07) of tobacco mosaic virus (TMV) were obtained from commercial orchids. These isolates comprise at least five distinct strains of TMV (01, 02, 04, 05, and 06). Of the five strains, four (01, 04, 05, and 06) are serologically indistinguishable from common TMV, but are distinct from each other and from other TMV strains when symptoms in various tobaccos and amino acid analyses are compared. The orchid strains of TMV differ from common TMV either by only a few amino acid exchanges, or by numerous amino acid exchanges. Strains 01 and 04 possess a pair of exchanges represented by Ile, Val --> Ala, Thr. If these strains are assumed to have arisen from type TMV by spontaneous mutational events, then the observed results suggest that the mutations arose through amination of uracil to cytosine. Strain 06 differs from common TMV by three exchanges, two of which include histidine and methionine residues; both of these amino acids are absent in common TMV. Unlike most orchid TMV strains, strain 02 (isolates 02 and 07) infects few tobaccos and differs from TMV by some 15 amino acid exchanges. It shares antigenic determinants, which are absent in common TMV, with HR and U2 strains of TMV.